1. Field of the Invention
The present invention relates generally to a method and arrangement for correctly estimating channel responses and more specifically to a method and arrangement which is capable of following rapid changes in channel characteristics. The present invention is well suited for use in mobile communications systems merely by way of example.
2. Description of the Prior Art
It is known in the art of a radio communications system that a receiver using maximum-likelihood sequence estimation exhibits extremely low error rate and particularly in the case of intersymbol interference. This maximum-likelihood sequence estimation technique is disclosed in an article published in IEEE Transactions on Information Theory, Vol. IT-18, No. 3, May 1972, entitled "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference" by G. D. Forney Jr. (paper 1).
In order to meet the situation wherein channel impulse responses vary with respect to time, an adaptive maximum-likelihood receiver has been proposed and disclosed in an article published in IEEE Transactions on Communications, Vol. Com-22, No. 5, May 5, 1974, entitled "Adaptive Maximum-Likelihood Receiver for Carrier-Modulated Data-Transmission Systems" by Gottfried Ungerboeck (paper 2). This type of arrangement or method utilizes a known training sequence, which is previously sent to a receiver in order to enable initial estimation of channel response of later data transmission. Following this, a decision sequence is generated from a sequence estimator with a certain delay using an adaptive algorithm. However, this algorithm has been unable to follow very rapid changes in channel characteristics.